Scheme of accelerated overcurrent protection for distribution systems with IDMT coordination

This paper presents the application of a new technique for accelerated overcurrent protection of distribution systems with inverse definite minimum time (IDMT) setting. In the technique, the sequence and super-imposed signals are used to determine the remote circuit breaker operation, from which whether a fault is inside the protected section can be derived and corresponding accelerated trip decision can be made. The technique has been implemented into the latest numeric relay, which takes into consideration of protection schemes with both fixed time and IDMT settings. The paper presents the test results with respect to a typical ring main distribution system with tapped-off-loads, which show that the relay is able to significantly increase the speed of response of the conventional IDMT time grading based protection scheme. (6 pages)